The present invention relates to a method for magnetic resonance imaging (MRI). It also relates to a device implementing this method.
Such a method or device can for example preferably allow a user to generate and use co-localized images, of the same spatial resolution, the same echo time (TE) and repetition time (TR) for nuclear magnetization, longitudinal (T1) and transverse (T2) relaxation times, the diffusion coefficient (D), the amplitude of the radiofrequency field (B1) and the inhomogeneities of the main magnetic field B0.
MRI is a clinical and pre-clinical imaging method used routinely in radiological medical diagnosis or for the assessment of animal models. Its non-invasive aspect and the multiple contrasts available make it indispensable in the assessment of numerous illnesses. A typical MRI examination contains a protocol for the acquisition of several imaging sequences, each having a different objective. The sequences called “proton density” sequences will provide information on the quantity of free water in the tissue. The sequences called “T1-weighted” generally serve for anatomical assessment, as they generate a good contrast between the biological tissues. The “T1-weighted” sequences also serve for visualizing contrast agents that can be injected during angiography or for tumor characterization. The sequences called “T2-weighted” have the overall property of making it possible to clearly distinguish the liquid contents and provide information for example on the presence of edemas. Further, contrast agents called T2 can be used in order to modulate the T2 weighting for some applications. The diffusion, which also influences the contrast of the images generated with particular sequences called “diffusion-weighted” are also used in order to extend the tissue characterization, for example in order to distinguish the viable areas from the necrotic areas in tumors; a distinction that is not always possible with the other types of weighting, or in order to carry out tractography of cerebral fibers based on the predominant water diffusion direction. Depending on the illness investigated and the anatomical area, several sequences are applied during an examination. These are applied sequentially, which as it is, requires a period of time for parametering for the electroradiological manipulators in order to adapt each sequence to the area under investigation. Due to constraints of acquisition time, of the weighting and localization type, the different items of information are generally not acquired with the same spatial resolution, do not cover the same fields of view and can even be affected by different spatial deformations, depending in particular on the different echo times and bandwidths of acquisition. There may be mentioned for example the need in some cases to resort to a subsequent correction of the images or to image retiming techniques in order to partially compensate for these effects. These aspects contribute to making MRI either limited to an interpretation based exclusively on visual comparisons, or introducing bias or imprecision factors resulting from correction algorithms. In order to improve diagnosis, there is therefore a need to be able to co-localize the different items of information exactly on the scale of the same imaging voxels.
Furthermore, the choice of the sequences applied and the parametering of the sequences is the responsibility of each radiology center, and the utilizations of the sequences may vary between manufacturers. There is no standard for a sequence and for the associated parameters applicable in all cases. These elements make multi-center comparisons of imaging results difficult, and slow down or even prevent a consensus being reached on the sequences and on the parametering thereof. In order to make diagnosis more reliable, the direct quantification of the physical parameters such as nuclear magnetization, T1 and T2, and the diffusion coefficient is a possibility that makes it possible to standardize the imaging results.
Further, in order to quantify the different parameters T1, T2 and the diffusion coefficient, multiple sequences exist, each adapted to the parameter to be quantified. There may be mentioned for example the inversion-recovery sequence with several inversion times for quantifying T1, the spin-echo sequence with several spin-echo times for quantifying T2, the diffusion-weighted spin-echo sequence for quantifying the diffusion coefficient. These different sensitization patterns can then be combined with different sequences, allowing localization with all the aforementioned drawbacks of spatial deformation between the acquisitions, of different field of view or area covered, reducing the possibility of pairing the extracted parameters voxel by voxel.
A standard rapid acquisition sequence is based on rapid-gradient echo. This imaging sequence consists of repeating a pattern every TR including a fixed-amplitude excitation, optionally combined with a gradient called a “selection” gradient, in order to carry out a spatial selection, followed by imaging gradients (for readout and phase encoding). Multiple variants of this sequence have been proposed. In order to synthesize the differences between the variants, the following elements will be noted:                the total compensation of the gradient areas between repetitions (sequence called balanced sequence), or the opposite, the non-compensation of this gradient area (sequence called non-balanced sequence). This last unbalanced variant creates different coherence orders that can optionally be exploited. A coherence order is denoted k. This is an integer that corresponds to the discrete Fourier series order that can be used in order to describe the configuration states of magnetization, as proposed and utilized in the state of the art.        The particular cycling of the phase θn of the radiofrequency pulse at repetition n without changing its amplitude with θn+1=θn+(n+1)Δ. The variants work either at fixed-phase as a function of the repetitions (Δ=0), or with a quadratic phase cycling (Δ≠0), equivalent to a constant and non-zero increment Δ from one repetition to another. Of course, Δ is defined modulo 2π.        The measurement between the pulses for 1 or more echoes, in particular corresponding to the phase coherence orders (k=0, generally the only one acquired, and/or k=−1 and/or k=1 which may also be the case.        The addition or not of additional gradients between the pulses in order to differently sensitize the diffusion        The acquisition of the signal during the transient phase or during the stationary phase. For the transient phase, the starting magnetization can be the state of thermal equilibrium or a state prepared using preparation patterns and the information used for the analysis is the variation in the signal as a function of the repetitions. In contrast, the stationary acquisition is based on the acquisition of a dynamic steady state reached following several repetitions.        
It should be noted here that the magnetization can be decomposed into a component aligned on the main magnetic field B0, called longitudinal component, and a transverse component, the latter capable of being detected by induction in a radiofrequency detector and capable of being spatially modulated with the gradients. It should be noted that the effect of a radiofrequency pulse applied to resonance and amplitude B1(t) of phase phi (with respect to an internal reference of the RF synthesizers) and applied for a duration T is to flip the magnetization of an angle α with B0, the angle being proportional to the integral of the RF field for the duration T, the axis of rotation itself being shifted by θ=phi+π/2 radians with respect to the frame of reference rotating at resonance. It should be noted that the longitudinal and transverse components relax with a characteristic time T1 and T2, respectively, this time being dependent on the local environment and possibly influenced by the magnetization transfer. Finally, it should be noted that the atoms or molecules carrying nuclear magnetization diffuse over time, which is described macroscopically by the Bloch-Torrey equations.
A significant complexity in the rapid sequences (generally with TR<T2), is that the transverse component that is dephased with the gradients and with the field inhomogeneities is “recycled” by the following excitation transferring a part longitudinally and reciprocally. This dephasing remaining between two pulses, combined with the non-suppression of the magnetization by the purely dispersive phenomena of relaxation and diffusion, is at the origin of different k states as previously mentioned. It is then difficult to describe the magnetization including the effects of relaxation, diffusion, angle, and a person skilled in the art often also reports that it is impossible to describe the effects, thus the limitation under certain restrictive conditions of validity or even confusion over the creation of coherence states when the phase is modulated, or finally the negation of diffusion effects.
The purpose of the present invention is to overcome at least one of the following problems of the state of the art:                proposing a new type of sequence or new forms of steady state, and/or        facilitating a determination, with respect to the Ernst angle, of the effect of the radiofrequency pulse producing the flip angle, and/or        determining in a quasi-simultaneous manner, several parameters from a nuclear magnetization, a flip angle of the magnetization, a diffusion coefficient, a longitudinal rate or time R1 or T1, and a transverse relaxation rate or time R2 or T2 of one and the same point in space, and/or        improving taking account of diffusion.        